The typical prior art exhaust muffler has comprised an array of generally parallel tubes supported by transversely extending elliptical or circular baffles. Selected portions of certain tubes typically have arrays of perforations or louvers extending therethrough. Some tubes in the array extend continuously from one baffle to the next, while other tubes may extend shorter distances. This typical prior art muffler further comprises a sheet of metal formed into a generally tubular shell having a cross section corresponding to the elliptical or circular shape of the baffles. The array of baffles and tubes are then slid into the tubular outer shell such that chambers are defined between the shell and adjacent pairs of baffles. The shell is then spot welded to the baffles.
At this point in the construction of the prior art muffler, opposed end caps or headers could b connected to opposed ends of the tubular shell to define a functional exhaust muffler. However, with this construction, it has been found that portions of the shell between adjacent baffles will vibrate in response to the flow of exhaust gases through the muffler. This vibration causes the outer shell of the prior art muffler to effectively function as a drum and produce a shell noise or ringing. To offset this shell noise, most prior art mufflers are provided with an outer wrapper which is wrapped around the tubular shell. The opposed end caps or headers are then mechanically joined to both the shell and the wrapper to complete the prior art muffler. The outer wrapper typically functions to dampen the shell noise.
Some prior art mufflers have been provided with arrays of embossments on the outer wrapper to define air pockets intermediate the outer wrapper and the shell to further contribute to this dampening of shell noise. Examples of mufflers having embossments in either the shell or the outer wrapper are shown in U.S. Pat. No. 2,661,073; U.S. Design Pat. No. 165,057; U.S. Pat. No. 2,151,084 and U.S. Pat. No. 2,835,336, all of which issued to Floyd E. Deremer.
Mufflers have been developed which are formed, at least in part, from stamp formed components. Certain of these prior art mufflers have a pair of plates stamp formed to define an array of parallel tubes. A plurality of transverse baffles support the internal plates and define walls of chambers within this prior art muffler. A conventional tubular outer shell is provided around both the baffles and the stamp formed tubes. A prior art muffler of this type is shown in U.S. Pat. No. 4,396,090 which issued to Wolfhugel on August 2, 1983. U.S. Pat. No. 4,396,090 does not suggest any structure for dealing with the above described shell noise, but presumably could employ the combination of a tubular shell and an outer wrapper as explained above.
Other prior art mufflers have been formed entirely with stamp formed components. Until recently, the stamp formed mufflers achieved relatively little commercial success compared to the above described mufflers with wrapped outer shells. Part of this poor commercial performance of the prior art stamp formed mufflers has been attributable to poor acoustical performance of most such mufflers. In particular, the typical prior art stamp formed muffler has been designed to permit expansion of exhaust gases and/or to achieve a convoluted movement of exhaust gases with a certain amount of noise attenuation associated therewith. However, these prior art stamp formed mufflers have provided little or no attenuation of residual low frequency sounds that cannot be attenuated adequately by expansion chambers, convolutions or the like. Thus, to the extent these prior art stamp formed mufflers were used at all, they typically would have been employed in environments and on vehicles that could have accepted the above described shell noise associated with the drum-like vibrations of the stamp formed muffler shells.
Certain of the fairly simple prior art stamp formed mufflers have considered merely of a pair of opposed outer shells stamp formed to define a convoluted flow path for the movement of exhaust gases. For example, U.S. Pat. No. 2,484,827 which issued to Harley on October 18, 1949, is directed to a muffler having two opposed stamped casing members. Each casing includes a planar peripheral flange. Ridges are formed in each casing, with the bottom of each ridge lying within the plane of the peripheral flange. Parallel corrugations extend out of the plane of the peripheral flange, with the corrugations of one casing being angularly aligned to the corrugations of the other. The casings are mounted together such that the peripheral flanges and ridges thereof are in face-to-face relationship and such that the corrugations extend in opposite directions. The corrugations thus define a single array of interconnected chambers through which exhaust gas may flow.
U.S. Pat. No. 3,176,791 issued to Betts et al. on April 6, 1965, and shows a muffler formed from a pair of opposed stamp formed or molded halves joined to one another along peripheral flanges which are generally planar for most of the body of the muffler. One half of the muffler shown in U.S. Pat. No. 3,176,791 includes a plurality of very deep corrugations, the bottoms of which are generally linear and lie in a plane slightly spaced from the plane of the peripheral flanges. These deep corrugations with the linear bottoms effectively define a plurality of separate chambers. The opposed half of the muffler shown in U.S. Pat. No. 3,176,791 includes a plurality of generally circumferentially extending inwardly directed arcuate grooves which terminate at arcuate ends generally tangent to the plane of the peripheral flange. Each inwardly directed groove effectively defines a continuous arcuate concavity across the entire width of each groove. Additionally, the spacing between adjacent grooves is many times greater than the width of each respective groove.
Substantial improvements to stamp formed mufflers have been developed as shown in U.S. Pat. No. 4,700,806 and U.S. Pat. No. 4,736,817, both of which issued to Jon Harwood and which are assigned to the assignee of the subject invention. The mufflers shown in U.S. Pat. No. 4,700,806 and in U.S. Pat. No. 4,736,817 provide the various advantages of stamp formed manufacturing including fewer components, lighter weight, lower costs and a manufacturing process that is well suited to a high degree of automation. However, and importantly, the mufflers shown in U.S. Pat. No. 4,700,806 and U.S. Pat. No. 4,736,817 can achieve noise attenuation that equals or exceeds the noise attenuation available with the conventional prior art mufflers with wrapped outer shells, and that far exceeds the acoustical tuning available with prior art stamp formed mufflers.
Further improvements to stamp formed mufflers are shown in co-pending applications that are assigned to the assignee of the subject invention. In particular, U.S. patent application Ser. No. 061,876, now U.S. Pat. No. 4,760,894 shows a stamp formed "EXHAUST MUFFLER WITH ANGULARLY ALIGNED INLETS AND OUTLETS". U.S. patent application Ser. No. 061,913, now U.S. Pat. No. 4,759,423, shows an efficient and effective "TUBE AND CHAMBER CONSTRUCTION FOR AN EXHAUST MUFFLER". U.S. patent application Ser. No. 106,244, now U.S. Pat. No. 4,765,437, is directed to a "STAMP FORMED MUFFLER WITH MULTIPLE LOW FREQUENCY RESONATING CHAMBERS" which enables plural stamp formed tuning tubes and plural low frequency resonating chambers with a significant reduction in the amount of deformation required in the stamp formed outer shells. U.S. patent application Ser. No. 146,032, now U.S. Pat. No. 4,821,840, is directed to a "STAMP FORMED MUFFLER WITH CONFORMAL OUTER SHELL" which enables the muffler to efficiently conform to the limited available space on a vehicle. The disclosures of the above identified patents and applications assigned to AP Industries, Inc. are incorporated herein by reference.
Referring to U.S. Pat. No. 4,700,806 and U.S. Pat. No. 4,736,817, it will be noted that a plurality of outwardly extending stamp formed reinforcing ribs are disposed on the major planar surfaces of the chambers formed in the external shell. Although ribs of this general construction contribute to a stiffening of the external shells, it is desirable to further prevent shell noise. Although separate wrappers or shells as used with conventional mufflers presumably could reduce shell noise, they would also add substantially to the weight and cost of the muffler. Similarly, a plurality of very deep creases as in the above referenced U.S. Pat. No. 2,484,827 and U.S. Pat. No. 3,176,791 also might reduce shell noise. However, these creases are difficult to form and make it difficult to achieve the acoustically required chamber volumes within the available space on the vehicle. Various patterns of outwardly convex arcuate ribs as shown in U.S. Pat. No. 4,700,806 and similar patterns of inwardly concave arcuate channels disposed on the major planar surfaces of the external shell have been experimented with. However, these various attempts have not adequately eliminated shell noise.
In view of the above, it is an object of the subject invention to provide a stamp formed muffler that effectively reduces shell noise to acceptable levels.
It is a further object of the subject invention to provide a stamp formed muffler having at least one external shell that is reinforced to substantially avoid or offset shell related noise.